USB: fsl_udc_core: prepare for SoCs with BE registers and descriptors

On some SoCs, the USB controller registers and descriptors can be big
or little endian, depending on the version of the chip. In order to be
able to run the same kernel binary on different versions of an SoC, the
BE/LE decision must be made at run time. Provide appropriate register
and descriptor accessors which are configurable at run time using the
configuration flags from fsl_usb2_platform_data data structure.

This is in preparation for adding support for MPC5121E DR USB2 Controller
to the FSL UDC driver.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Li Yang <leoli@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>